For the assembly of a motor vehicle wheel, usually hub bearing assemblies with a double-row of rolling-contact elements interposed between the respective inner and outer raceways are used. The two radially outer raceways are both formed by the same outer bearing ring. The two radially inner raceways are formed by two respective annular steel bodies, integral in rotation to a steel hub that forms a cylindrical portion extending in an axially inner direction, and a flange that extends in a radially outer direction allowing to assemble the wheel. In the hub-bearing units of the so-called third generation, the steel hub directly forms the inner raceway for the row from the outboard side. The inner raceway for the row of rolling-contact elements from the inboard side, however, is made by a steel ring formed separately by the hub, also known as small inner ring, and fitted on the cylindrical portion of the hub. In the hub-bearing units of the so-called second generation, the inner raceways are formed by two respective steel rings (small inner rings) fitted axially aligned on the cylindrical portion of the hub. The cylindrical portion of the hub has a tubular end that protrudes over a radial surface at the axially inner end of the inner ring located on the axially inner side. This tubular end undergoes a cold deformation, typically by orbital roll forming, in a radially outer direction; a rolled, plastically deformed edge is thus obtained, which axially locks the second ring and axially preloads the entire bearing unit. The machines performing the orbital roll forming must be able to apply a very high force to the forming tool, generally greater than 100,000 N.
Therefore, an object of the invention is to reduce the force required to perform the orbital roll forming, possibly using less efficient machines but facing lower costs.
It is known that the raceways must be hard enough to withstand the Hertzian stresses of the rolling contact. For this reason, in a third generation hub bearing assembly it is necessary for the hub to undergo induction hardening treatment in order to harden only the raceway area. The hardened area must not extend as far as the end portion to be rolled, since this must not become too hard and brittle, but must instead remain suitable for undergoing plastic processing. Another object of the invention is therefore, to eliminate the costs associated with the need to carry out traditional thermal hardening treatment by induction.
A further object of the invention is to reduce the overall weight of a hub bearing unit. In the automotive industry there is an ever increasing demand in terms of reduction of the weight of motor vehicle component parts in order to reduce fuel consumption and exhaust emissions. In order to reduce the overall weight of the wheel and, in particular of the rotating mass, in recent years hub bearing assemblies have been proposed having a rotating flanged ring made of two different materials, joined together in a single piece. In such rings, a tubular core made of a first material of high toughness, such as steel for bearings, forms the raceways, and a second light material, such as a light metal, forms the remaining part of the ring, including an outer flange for mounting the wheel. See for example the patent publication WO 2008/147284 A1. In some cases, the coupling between the steel core and the lighter flange is made by form coupling with interference. These couplings do not always prove to be long-lasting, especially after prolonged use. Indeed, the different coefficients of thermal expansion of steel and aluminum tend to cause the two materials to separate one from the other. In other cases, the coupling is obtained by molding or casting the light material, for example an aluminum alloy, over the tubular steel core. To avoid or limit movements between the two materials, in rings of this type, the two materials are joined with complexly-shaped interface surfaces, so as to produce undercuts that behave as joints between the two materials. The costs of producing rings using this technology are rather high.